cassandra-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Peer, Oded" <Oded.P...@rsa.com>
Subject RE: Updating only modified records (where lastModified < current date)
Date Wed, 13 May 2015 12:02:12 GMT
USING TIMESTAMP doesn’t avoid compaction overhead.
When you modify data the value is stored along with a timestamp indicating the timestamp of
the value.
Assume timestamp T1 < T2 and you stored value V with timestamp T2. Then you store V’
with timestamp T1.
Now you have two values of V in the DB: <V,T2>, <V’,T1>
When you read the value of V from the DB you read both <V,T2>, <V’,T1>, Cassandra
resolves the conflict by comparing the timestamp and returns V.
Compaction will later take care and remove <V’,T1> from the DB.

I don’t understand the ETL use case and its relevance here. Can you provide more details?

UPDATE in Cassandra updates specific rows. All of them are updated, nothing is ignored.


From: Ali Akhtar [mailto:ali.rac200@gmail.com]
Sent: Wednesday, May 13, 2015 2:43 PM
To: user@cassandra.apache.org
Subject: Re: Updating only modified records (where lastModified < current date)

Its rare for an existing record to have changes, but the etl job runs every hour, therefore
it will send updates each time, regardless of whether there were changes or not.

(I'm assuming that USING TIMESTAMP here will avoid the compaction overhead, since that will
cause it to not run any updates unless the timestamp is actually > last update timestamp?)

Also, is there a way to get the number of rows which were updated / ignored?

On Wed, May 13, 2015 at 4:37 PM, Peer, Oded <Oded.Peer@rsa.com<mailto:Oded.Peer@rsa.com>>
wrote:
The cost of issuing an UPDATE that won’t update anything is compaction overhead. Since you
stated it’s rare for rows to be updated then the overhead should be negligible.

The easiest way to convert a milliseconds timestamp long value to microseconds is to multiply
by 1000.

From: Ali Akhtar [mailto:ali.rac200@gmail.com<mailto:ali.rac200@gmail.com>]
Sent: Wednesday, May 13, 2015 2:15 PM
To: user@cassandra.apache.org<mailto:user@cassandra.apache.org>
Subject: Re: Updating only modified records (where lastModified < current date)

Would TimeUnit.MILLISECONDS.toMicros(  myDate.getTime() ) work for producing the microsecond
timestamp ?

On Wed, May 13, 2015 at 4:09 PM, Ali Akhtar <ali.rac200@gmail.com<mailto:ali.rac200@gmail.com>>
wrote:
If specifying 'using' timestamp, the docs say to provide microseconds, but where are these
microseconds obtained from? I have regular java.util.Date objects, I can get the time in milliseconds
(i.e the unix timestamp), how would I convert that to microseconds?

On Wed, May 13, 2015 at 3:56 PM, Ali Akhtar <ali.rac200@gmail.com<mailto:ali.rac200@gmail.com>>
wrote:
Thanks Peter, that's interesting. I didn't know of that option.

If updates don't create tombstones (and i'm already taking pains to ensure no nulls are present
in queries), then is there no cost to just submitting an update for everything regardless
of whether lastModified has changed?

Thanks.

On Wed, May 13, 2015 at 3:38 PM, Peer, Oded <Oded.Peer@rsa.com<mailto:Oded.Peer@rsa.com>>
wrote:
You can use the “last modified” value as the TIMESTAMP for your UPDATE operation.
This way the values will only be updated if lastModified date > the lastModified you have
in the DB.

Updates to values don’t create tombstones. Only deletes (either by executing delete, inserting
a null value or by setting a TTL) create tombstones.


From: Ali Akhtar [mailto:ali.rac200@gmail.com<mailto:ali.rac200@gmail.com>]
Sent: Wednesday, May 13, 2015 1:27 PM
To: user@cassandra.apache.org<mailto:user@cassandra.apache.org>
Subject: Updating only modified records (where lastModified < current date)

I'm running some ETL jobs, where the pattern is the following:

1- Get some records from an external API,

2- For each record, see if its lastModified date > the lastModified i have in db (or if
I don't have that record in db)

3- If lastModified < dbLastModified, the item wasn't changed, ignore it. Otherwise, run
an update query and update that record.

(It is rare for existing records to get updated, so I'm not that concerned about tombstones).

The problem however is, since I have to query each record's lastModified, one at a time, that's
adding a major bottleneck to my job.

E.g if I have 6k records, I have to run a total of 6k 'select lastModified from myTable where
id = ?' queries.

Is there a better way, am I doing anything wrong, etc? Any suggestions would be appreciated.

Thanks.




Mime
View raw message